In a plastic extrusion process, solid plastic material in the form of powder or pellets is fed through a hopper into a cylindrical barrel of an extruder. The barrel which is divided into a plurality of heating zones is heated to a temperature above the melting or flow point of the plastic material. A screw which is rotatably mounted in the barrel is turned to move the plastic material through a feed zone, a compression zone and a metering zone toward a crosshead. The heated walls and frictional heat from the rotating screw cause the plastic material to change from the solid state to the molten state. As a result, a melt pressure is developed. The molten plastic material is then forced through a die in the crosshead which forms it into a desired shape. For example, the plastic material which is flowed through the die may be used to insulate conductors or to jacket cables which are moved through the crosshead.
The extruder compresses the plastic material into a compact solid bed. The term "solid bed" refers to the plastic material prior to a transformation into a substantially less viscous melt material. The advancing flight of the screw forces melted plastic material downwardly along the flight surface and into a circulating melt pool at the rear of a channel between turns of the flight. As the plastic material is moved along the screw channel, the width of the melt pool increases and the width of the solid bed decreases. At some location in the compression section, the solid bed breaks up. As this occurs, the plastic material is transformed into a viscous melt surrounded by less viscous melt. By the time the material has been advanced into the metering section, it should be thoroughly mixed and thermally homogenized.
Output rates for extrudates which cover conductors and which have relatively thin cross sections depend on the ability to control various operating parameters. One such parameter is the melt temperature which is the temperature of the molten material at the outlet of the barrel. Another is the melt pressure which is the pressure of the molten material. The pressure may be controlled by controlling the speed of the extruder screw whereas the melt temperature is controlled by controlling the temperature of each zone. Generally, the lack of temperature uniformity which manifests itself by defects such as nonuniform dimensions or reduced strength characteristics evinces a failure to achieve a thorough mixing of the plastic material within the extruder.
Quality control becomes more difficult when extruding vinyl compositions. Polyvinyl chloride is a relatively inexpensive versatile polymer that can be compounded and extruded as a coating for communication type cable and wire products. It is more flame retardant than polyethylene and, therefore, is highly desirable for indoor cable and wire applications.
Polyvinyl chloride compositions which are used for extrusion generally include not only the basic resin but also other constituents or additives. These include lubricants, stabilizers, pigments, plasticizers, fillers, flame retardants and smoke suppressants.
Additives have a pronounced effect on the processing of the vinyl composition. For example, it has been found that vinyl compositions fuse at various points along the length of the screw. This occurs because of different amounts of shear heat energy that have been imparted to the composition during its compounding. Secondly, this occurs because of the different constituents and/or the different percentage of different constitutents that have been added into the composition. All the constituents contribute to the fusion behavior of the composition. This problem is not experienced when processing polyethylene which is widely used for insulating conductors inasmuch as it is a purer material and is not contaminated by the constituents which are used to impart various properties to a vinyl composition.
Uncontrolled fusion is troublesome because if the material fuses at points too far along the screw toward the crosshead, it is possible to have unmelted material being moved to the die. This may cause the insulation being extruded over a substrate such as a conductor to have a rough outer surface. Such an insulation is difficult to surface color for purposes of identification. This problem is magnified if the screw is being rotated at a relatively low speed.
One solution to this problem has been to compound the polyvinyl chloride material at a lower rate, which of course results in a lower output. Or, the temperature of the composition may be controlled as it is being compounded. However, if the composition is compounded at too low a temperature, then the resin may not be fully fluxed nor fully homogenized. This makes it difficult to reuse the composition in the extruder without substantial working. On the other hand, it is not desirable to have a composition in pellet form that is fused so well that it can't be remelted easily and processed through the extruder. Further, control of the compounding by an end user may not be feasible.
Seemingly, the prior art is not concerned with the fusion of the plastic pellets at different points along the barrel. U.S. Pat. No. 4,197,070 typifies the art. Therein is shown a melt pressure transducer which is positioned near the output end of the barrel adjacent to the tip of the screw and which is used to control the the melt pressure and temperature of the plastic material at the outlet of the barrel. Also, it is common to position temperature sensors along the barrel. However, temperature readings are not indicative of the extent of the melt. Partially melted or completely melted plastic material in the barrel could exhibit the same temperature at the same screw speed.
Still of concern and still not addressed by the prior art is the control of the location along the barrel where the plastic material fuses. This is particularly important in extruding vinyl products because vinyl compositions have different fusion characteristics depending on their heat histories. Without a solution to this problem, pellets received from a compounder may not be fully fluxed by the end user in an extruder. What is needed and what is not available in the prior art is a method of controlling a vinyl plastic composition at the user stage in order to insure a uniformly homogenous extrudate.